Thanks for the responses so far. The car is an '01 2.5RS with about 70K miles. Fuel filter, spark plugs, and lots more are new.
I guess the fuel pressure test would be the obvious thing to do -- not exactly easy but I'll give it a try.
An exhaust leak wouldn't normally cause this kind of lean situation, right?
Anyway, thanks for the thoughts -- I'll keep the board updated with where this is going.